The market for agents that benefit animal health includes feed enzymes, feed additives, nutraceuticals, small molecules, and protein therapeutics, such as vaccines, antibodies, enzymes, and peptides. Annually, over $13.4 billion is spent on agents to promote animal health, with more than half of that amount going to poultry, fish, and livestock e.g., cattle, swine, and sheep. Vaccines to protect against pathogens account for a large portion of the animal health market.
Eukaryotic (protozoan), viral, and bacterial pathogens afflict poultry, fish, and livestock, and can have devastating effects and impose significant economic problems on the poultry, fish and livestock industries. Accordingly, there is a need to develop compositions such as vaccines that are effective in protecting animals susceptible to pathogenic/disease-causing microorganisms.
Both humoral and cellular immunity are recognized as important in developing protection against several of the diseases derived from pathogenic microorganisms that afflict poultry, fish and livestock. Traditionally, vaccines have been based on live attenuated, or inactivated pathogens. However, these strategies are inefficient due to factors such as antigenic variability of pathogens such as viruses. Peptide vaccines that consist of antigenic peptides or peptide fragments of pathogens have been developed. Conserved peptide fragments are less likely to exhibit antigenic variability, and can overcome some of the problems associated with traditional peptides. Accordingly, subunit vaccines have been developed that target conserved regions of pathogens. However, synthetic peptide vaccines tend to be poorly immunogenic, and also tend to induce humoral antibody responses, but are less able to induce cell-mediated responses. Multivalent subunit vaccines, which contain both immunodominant B-cell and T-cell epitopes also address the above problems.
Effective vaccines generate effective memory responses for both humoral and cell-mediated immunity. This requires generation of a population of memory Th cells. By both directly contacting lymphokines and by secreting lymphokines such as IL2 and IL4, Th cells promote and support the expansion and differentiation of T and B cell precursors into effector cells.
Th cells, via T-cell receptors, recognize a complex formed between Class II MHC molecules and antigenic peptides (T-cell epitopes). Peptides containing epitopes recognized by Th cells are useful in inducing immune responses. Different class II MHC molecules are capable of binding to and presenting different peptides to Th cells. The genetic loci encoding MHC molecules are the most polymorphic known in higher vertebrates, resulting in a tremendous diversity of MHC proteins within a species. The differences between the different haplotypes (or unit of inheritance of MHC molecules) in turn influence the cells' ability to recognize a given peptide. Consequently, individuals differ in their ability to generate an immune response to any given pathogen. Accordingly, the identification of polypeptides that bind several MHC class II molecules is useful in the development of peptide vaccines that can induce an effective immune response in a wide variety of individuals.
Investigators have identified “promiscuous T-cell epitopes” (“PTCEs”) in humans. See, U.S. Pat. No. 6,143,935; U.S. Pat. No. 6,143,517; U.S. Pat. No. 6,689,363. Promiscuous T-cell epitopes (or “PTC epitopes”) are highly immunogenic peptides that can be characterized in part by their capacity to bind several isotypic and allotypic forms of human MHC class II molecules. By helping to bypass MHC restriction, they can induce T-cell and antibody responses in members of a genetically diverse population expressing diverse MHC haplotypes. The PTC epitopes can therefore be combined with antigens that, by themselves, are poorly immunogenic, to generate potent peptide immunogens.
The Clostridium tetani tetanus toxin P2 and measles virus fusion protein epitopes have been established as strong PTCEs that are highly immunogenic in human and murine models (Demotz, et al., (1989) J Immunol. 15; 143(12):3881-6; Panina-Bordignon, et al. (1989) Eur J Immunol.; 19(12):2237-42; Partidos and Steward, (1990), J Gen Virol.; 71 (Pt 9):2099-105. U.S. Patent Publication No. 2004/0086524 describes fusion proteins between a tetanus toxin P2 PTCE or a measles virus PTCE and the OspA protein of the bacterium Piscirickettsia salmonis, a bacterial pathogen of salmonid species. The presence of the tetanus toxin PTCE or the measles virus PTCE augmented the antibody response to the OspA protein, and enhanced lymphocyte proliferation in response to the vaccine, and conferred protection against the pathogen.
To date, PTCEs that are effective in augmenting immune responses in poultry have not been identified. Further, the need exists to provide improved vaccines against protozoan pathogens. Finally, the discovery of novel PTCEs that are effective in augmenting immune responses in poultry, livestock and fish is desirable.